Different story if you designed, built and installed a piece of equipment that failed, however simply maintaining someones tank would not leave you liable if something failed, even if it killed someone. Even if you purposely smashed the tank unless they could physically prove you did so it would just come down to equipment failure and provided you didn't build the tank you're 100% not liable. I had decent coverage as well but it was because I was building and installing equipment, if it was strictly maintenance I wouldn't have bothered.

Nate are you incorporated? I learned a lot when I was in the business and you might be surprised what you're actually covered for. For example, any plumbing you actually should be licensed, any electrical same deal. Anything warranting high structural load may require engineering approval. If **** really hits the fan hard your insurance company will likely just pull out. This is partially why I don't do the business anymore, too much risk.